Why Must We Treat a Dead Body with Respect?
The Halacha distinguishes between Tashmishei Mitzvah (items with which a person does a mitzvah, e.g. – Tsitsis, Lulav, Succah, etc.) and Tashmishei Kedusha (items which involve Scriptural writing, e.g. – Sefer Torah, Tefillin, Mezuzah, etc.). After Succos, a person may theoretically throw out his Lulav and Esrog. The law is that Tashmishei Mitzvah do not retain sanctity, and after the completion of their use for the mitzvah performance, a person may discard them. (There is a nice custom to use the Lulav to burn the Chometz before Pesach, but this is only a custom and not a fundamental requirement.) Tashmishei Kedusha, however, remain eternally holy – even after the completion of their service in the performance of a mitzvah. Even if a Sefer Torah or Tefillin fall into water and become ruined, once they had sanctity, the sanctity remains forever.
The law of Tashmishei Kedusha prevails to such an extent that even items used as accessories to an item containing Torah pesukim (such as cloth coverings for Sifrei Torah) have sanctity even after they have become worn out, and must be buried rather than simply discarded.
What is the difference? The difference is that by Tashmishei Mitzvah, once the mitzvah is finished there is no more holiness. However, when the Name of G-d and words of Torah are written on parchment, the holiness remains forever.
This, he explains, is why a human body has sanctity even after death. Every Jew is a vehicle for Words of Torah. Just as parchment becomes holy forever, so too a Jew who spoke words of Divrei Torah, who said the words Shema Yisrael—his body now becomes like a Sefer Torah. Therefore, despite the fact that the soul has left, the words of holiness that were “inscribed” on that body during his lifetime put an eternal Kedusha on that body, for ever and ever. (adapted from Rabbi Yissocher Frand on torah.org)